[0001] This invention relates to a display apparatus which employs a plurality of light
emitting elements such as organic electro-luminescence elements that are controlled
their intensity by currents flowing through each picture element. This invention is
particularly relates to a display apparatus of so-called an active matrix type display
apparatus in which amount of current supplied to each light emitting element is controlled
with active elements such as insulated gate type field effect transistors equipped
in each picture element. And this invention further relates to a drive circuit to
be applied to such active matrix type display apparatus, wherein leakage current of
sub-threshold level flowing through the insulated gate type field effect transistors
is effectively suppressed. Generally, in a picture display apparatus of an active
matrix type, a plurality of picture elements are arranged in a matrix form, and a
video image is displayed by controlling intensity of each picture element according
to given intensity information of the video image. Transmission factor of each picture
element changes according to an applied voltage to each picture element when a liquid
crystal device is used as an electro-optic material. Also in the picture display apparatus
of the active matrix type employing organic materials as the electro-optic materials,
the operation thereof is similar to the operation of the liquid crystal device. However
different from the liquid crystal display, an organic EL (Electro-Luminescence) display
is so-called self-radiation type display having a light emitting device at each picture
element, so that the EL display has advantages over the liquid crystal device as follows.
Namely, a visibility of a video image is higher, a back-light is not necessary and
a response speed thereof is faster than that of the liquid crystal display. Intensity
of the individual light emitting device of the organic EL (Electro Luminescence) display
is controlled by the amount of drive current. Namely, the organic EL display is greatly
different from the liquid crystal display in the point that the light emitting device
is a current control type or a current drive type element.
[0002] Same as the liquid crystal display, the organic EL display can possibly take both
a simple matrix type and an active matrix type as the drive system. In the simple
matrix type drive system, a construction thereof is simple, but it is difficult to
apply a large-scale display and a high definition display. Accordingly the development
for the active matrix system is more active than for the simple matrix type system.
In the active matrix system, the current flowing through the light emitting device
of each picture element is controlled with an active element (Thin Film type Transistor
(TFT) which is one of an insulated gate type field effect transistor) fabricated in
the picture element. An example of one picture element in the organic EL display of
this active matrix system is depicted in Fig. 6 as an equivalent circuit. Each picture
element comprises of a light emitting device OLED, a first thin film transistor TFT
1, a second thin film transistor TFT 2 and a retention capacitor C. The light emitting
device is an organic electro-luminescence (EL) element. The most of the organic Electro-luminescence
device has a rectification characteristic so that the EL element can be called as
an OLED (Organic Light Emitting Diode) device, and in this Fig. 6, a sign of a diode
device is applied to a sign for the light emitting device OLED. The light emitting
device is not limited to the OLED device, and another type light emitting element
can be applied if an intensity of such element is controlled by the drive current
flowing through the element. In addition, as the light emitting device, the rectification
characteristic is not always demanded. In the figure, a source electrode of the P-channel
type transistor TFT 2 is connected to a Vdd (power potential), a cathode electrode
of the light emitting device OLED is connected to ground potential and an anode electrode
of the light emitting device OLED is connected to a drain electrode of the P-channel
type transistor TFT 2. On the other hand, a gate electrode of the N-channel type transistor
TFT 1 is connected to a scanning line SCAN, a source electrode thereof is connected
to a data line DATA and a drain electrode thereof is connected to both the retention
capacitor C and a gate electrode of the transistor TFT 2.
[0003] At first the scanning line SCAN is made in selected status in order to drive the
picture element, then a data potential (signal voltage) Vw representing an intensity
information is given to the data line DATA. Then the transistor TFT 1 is made ON,
thereby the retention capacitor C charges or discharges and a gate potential of the
transistor TFT 2 becomes to the data potential Vw. After that the scanning line SCAN
is made in non-selected status, and the transistor TFT 1 is accordingly made OFF.
In this case, the transistor TFT 2 is separated electrically from the data line DATA,
but the gate potential of the transistor TFT 2 is maintained in stable by virtue of
the retention capacitor C. A current flowing through the light emitting device OLED
by way of the transistor TFT 2 corresponds to a value of a gate-source voltage Vgs
of the transistor TFT 2, so that the light emitting device OLED continues to emit
light with the intensity corresponding to the current amount supplied through the
transistor TFT 2.
[0004] By the way, a current Ids flowing between the drain-source of the transistor TFT
2 is a drive current to be supplied to the light emitting device OLED. When the transistor
TFT 2 works in saturation range, the drive current Ids is shown with a following expression.
[0005] Where the Cox is a gate capacitance of an unit area, and the Cox is given with a
following expression.
[0006] In these expressions (1) and (2), the Vth shows a threshold voltage of the transistor
TFT 2, the µ shows a mobility of a carrier, the W shows a channel width, the L shows
a channel length, the ε 0 shows an electric constant, the ε r shows a relative permittivity
of a gate insulator film and the d is a thickness of the gate insulator film.
[0007] According to the expression (1), the drive current Ids can be controlled by the data
potential Vw to be applied to the picture element. As a result, the intensity of the
light emitting device OLED can be controlled in accordance with the drive current
Ids. The reason for operating the transistor TFT 2 in the saturation range is explained
as follows. Namely the drive current Ids is controlled only by the gate-source voltage
Vgs of the transistor TFT 2 in the saturation range, and the drive current Ids does
not depend on the drain-source voltage Vds of the transistor TFT 2. Namely, even if
the drain-source voltage Vds of the transistor TFT 2 changes by characteristic dispersion
of the light emitting device OLED, a predetermined amount of the drive current Ids
can be stably supplied to the light emitting device OLED.
[0008] As above described, in the circuit structure of the picture element as shown in Fig.
6, once the light emitting device OLED is supplied the signal voltage Vw, the light
emitting device OLED continues to emit light with a constant intensity during one
scan cycle (one frame) until the writing voltage is renewed next. As shown in Fig.
7, the active matrix type display apparatus is constituted by arranging a plurality
of the picture elements such as depicted in Fig. 6 in a matrix form. In the conventional
active matrix type display apparatus as shown in Fig. 7, scanning lines SCAN-1 to
SCAN-N for selecting one picture element 25 with a predetermined scanning cycle (one
frame of the NTSC standard) and data lines DATA for giving a intensity information
(the data potential Vw) to one picture element 25 are arranged in a matrix form. The
scanning lines SCAN-1 to SCAN-N are connected to a scanning line drive circuit 21,
and data lines DATA are connected to a data line drive circuit 22.
[0009] A desired video image can be displayed by repeating the supply of the data potential
Vw through the data lines DATA by the data line drive circuit 22 while selecting scanning
lines SCAN-1 to SCAN-N by the scanning line drive circuit 21. In a simple matrix type
display apparatus, the light emitting device emits light at the moment when selected,
but in a active matrix type display apparatus as shown in Fig. 7, the light emitting
device of each picture element 25 continues to emit light even after finishing the
selection, thereby a total amount of the drive current can be reduced in the active
matrix type display apparatus compared with the simple matrix type display apparatus
and this becomes profitable with a display apparatus of, in particular, a large-sized
and a high definition type. Generally, in the active matrix type organic EL display,
a TFT (Thin Film Transistor) device formed on the glass substrate is utilized as an
active element, and this depends on the next reason. Namely, as the organic EL display
is a direct viewing type display, the size of the display becomes comparatively large.
Therefore it is not realistic to use a single crystal silicon substrate for fabricating
an active element for the display due to a production cost or constraint of production
facility.
[0010] Accordingly in the active matrix type organic EL display, a comparatively large-sized
glass substrate is used, and it is normal that the TFT device that it is comparatively
easy to form on the glass substrate is used as an active element. However, amorphous
silicon and poly-silicon used for fabricating the TFT device show bad crystallization
characteristics compared with single crystal silicon and controllability of conduction
mechanism is bad, so that fabricated TFT device shows relatively large dispersion
of characteristic. Particularly in the case where a poly-silicon TFT device is formed
on a relatively large-sized glass substrate, a laser annealer is usually employed
in order to avoid a problem of heat transformation of the glass substrate. But in
this case, it is difficult to uniformly irradiate laser energy on the large-sized
glass substrate, so that dispersion by the place for crystalline condition of poly-silicon
is not avoided.
[0011] As a result, the threshold voltage Vth of the TFT device for a picture element shows
a dispersion of several hundreds mV, or even more than 1V among the TFT devices formed
on the same substrate. In this case, even if, for example, the same signal voltage
Vw is supplied to different picture elements, the drive current Ids flowing through
the each OLED device differs from desired value depending on the aforesaid expression
(1) due to the dispersion of the threshold voltage Vth of the TFT device, so that
as a result it can not be expected to obtain a a display apparatus of high picture
quality at all. This can say about dispersion of a carrier mobility µ and each parameter
of the expression (1) are similar in addition to the threshold voltage Vth. In addition,
the dispersion of each parameter as mentioned above is affected not only by the dispersion
between the picture elements, but also affected by fabrication lot, every manufacturing
lot or every product to some extent. In this case, it is necessary to decide setting
for the signal voltage Vw in order to flow desired drive current Ids according to
the completion of a product based on the parameters of the expression (1). But this
is not only unrealistic in mass production process of the display apparatus, but also
very difficult to take measures to meet the situation for the change in characteristic
drift of the TFT device by environmental temperature and change in properties with
time for the TFT device produced by activity of a long term use.
[0012] A display apparatus of the present invention comprises a scanning line drive circuit
for sequentially selecting scanning lines, a data line drive circuit which contains
a current source for generating signal current having current level corresponding
to an intensity information and for supplying thus generated signal current sequentially
to the data lines, and a plurality of picture elements each having a light emitting
device of current drive type which emits light with supply of drive current, wherein
the picture element is provided at each cross point of the data line and the scanning
line. Each picture element comprises an accept section for accepting signal currents
from corresponding data line when selected, a converting section for converting accepted
signal current once into corresponding voltage level and restoring the voltage level
and a drive section for supplying a drive current having current level corresponding
to the restored voltage level to the corresponding light emitting device. Further
the converting section includes the conversion thin film transistor having a gate
electrode, a source electrode, a drain electrode and a channel and a capacitor connected
to the gate electrode of the transistor. The above mentioned conversion thin film
transistor generates at the gate electrode the voltage level converted by flowing
through the channel the signal current taken through the accept section and the capacitor
holds the voltage level generated at the gate electrode. Furthermore, the above mentioned
drive section contains the drive thin film transistor including a gate electrode,
a drain electrode, a source electrode and a channel and the drive thin film transistor
supplies, through the channel, the drive current to the light emitting device, wherein
the drive current has the current level corresponding to the voltage level stored
in the capacitor and accepted at the gate electrode of the transistor. A threshold
voltage of the drive thin film transistor is set not to become lower than a threshold
voltage of the conversion thin film transistor corresponding to the picture element.
To be concrete, a gate length of the drive thin film transistor is set not to be shorter
than a gate length of the conversion thin film transistor. Or thickness of a gate
insulator of the drive thin film transistor may be set not to be thinner than thickness
of a gate insulator of the conversion thin film transistor corresponding to the picture
element. Further the threshold voltage of the drive thin film transistor may be set
not to be lower than the threshold voltage of the conversion thin film transistor
corresponding to the picture element by adjusting impurity density injected in the
channel of the drive thin film transistor. Preferably the drive thin film insulated
gate type field effect transistor works in saturation range and supplies drive current
corresponding to the difference between the threshold voltage and the voltage level
given to the gate electrode into the light emitting device. Further a current mirror
circuit is constituted by directly connecting the gate electrode of the drive thin
film transistor to the gate electrode of the conversion thin film transistor, so that
the current level of the signal current and the current level of the drive current
are made to be a proportional relation. Further above mentioned accept section includes
the switch thin film transistor interposed between the drain electrode and the gate
electrode of the conversion thin film transistor and this switch thin film transistor
is made ON when the current level of the signal current is converted into the voltage
level, and generates at the gate electrode of the conversion thin film transistor
a voltage level referenced with the source electrode by electrically connecting the
gate electrode and the drain electrode of the conversion thin film transistor. Preferably
the organic electro-luminescence device (OLED) is employed as the light emitting device,
and a thin film transistor (TFT), in which the source, the drain and the channel are
formed with poly-crystal semiconductor thin films, is employed as the drive insulated
gate type field effect transistor (FET) and the conversion insulated gate type field
effect transistor.
[0013] The hereinafter described embodiment of the invention presents a display apparatus
capable of displaying a high quality image by supplying desired drive current to a
light emitting device of each picture element in stable and precision in spite of
characteristic dispersion of each active device of the picture element. In particular
by suppressing leakage current ofsub-threshold level flowing through atransistor TFT
(Thin Film Transistor) which drives an OLED (Organic Light Emitting Diode) device,
the drive circuit of the invention prevents slight luminescence of the picture element
by the leakage current.
[0014] A picture element drive circuit of this invention preferably has following features.
Firstly a writing of the intensity information to the picture element is done by supplying
the signal current corresponding to the intensity into the data line and the signal
current flows through the source-drain of the conversion insulated gate type field
effect transistor in the picture element, and thereby generates a gate-source voltage
corresponding to the signal current. Secondly, thus generated gate-source voltage
or the gate voltage is retained in a operation of a capacitance formed in the picture
element or a capacitance existing parasitically and is kept within a predetermined
interval even after the completion ofthe writing of the intensity information to the
picture element. Thirdly, the current flowing through the OLED device is controlled
by the conversion insulated gate type field effect transistor oneself connected thereto
in series or the drive insulated gate type field effect transistor which is in addition
provided in the picture element and the gate electrode thereof is connected to the
gate electrode of the conversion field effect transistor. In this case, the gate-source
voltage upon driving the OLED device is approximately equal to the gate-source voltage
of the conversion field effect transistor generated due to the above described first
feature. Fourthly, the data line and the picture element is connected by the input
insulated gate type field effect transistor which is controlled by a first scanning
line and the gate-drain of the conversion insulated gate type field effect transistor
is short-circuited by the switch insulated gate type field effect transistor controlled
by a second scanning line. Namely by summing up above described features, the most
important feature is that the intensity information is given in the form of voltage
value in the conventional case, but the intensity information is given in the form
of current value, namely it is current writing type in the display apparatus of the
present invention.
[0015] The reason why the problem of causing a desired current to flow precisely to the
OLED device in spite of characteristic dispersion of the transistor TFT can be achieved
by the first to fourth features will be explained next. In the following explanation,
the conversion insulated gate type field effect transistor is called as a transistor
TFT 1, the drive insulated gate type field effect transistor is called as a transistor
TFT 2, the input insulated gate type field effect transistor is called as a transistor
TFT 3 and the switch insulated gate type field effect transistor is called as a transistor
TFT 4. But in the present invention, these transistors are not limited to thin film
transistors, and an insulated gate type field effect transistor such as a single crystal
silicon transistor made on a single crystal silicon substrate or a SOI (Silicon On
Insulator) substrate can broadly adopt as an active element of the present invention.
By the way, when writing the intensity information, signal current to flow in the
transistor TFT 1 is defined as a signal current Iw and as a result, voltage between
the gate and the source electrodes of the transistor TFT 1 is defined as a voltage
Vgs. The transistor TFT 1 works in the saturation range because the gate and drain
electrodes of the transistor TFT 1 are short-circuited by the transistor TFT 4 during
writing operation. Thereby the signal current Iw is given with a following expression.
[0016] Denotations of each parameter follow in case of aforesaid expression (1). When current
flowing through the OLED is defined as a Idrv, the Idrv is controlled the current
level by the thin film transistor TFT 2 connected to the OLED device in series. In
this invention, the gate-source voltage of the transistor TFT 2 is becomes the voltage
Vgs in the expression (3), so that a following expression is established if the transistor
TFT 2 works in its saturation range.
[0017] Denotations of each parameter follow it in case of aforesaid expression (1). Incidentally
a condition for the thin film transistor of a insulated gate field effect type working
in its saturation range is given with a following expression as drain-source voltage
of the thin film transistor is a voltage Vds.
[0018] The transistors TFT 1 and TFT 2 are formed close to each other within a small picture
element, so that approximately µ1=µ2 and Cox1 = Cox2, and accordingly it is thought
to be Vth 1 = Vth 2 so long as no-particular idea is introduced in fabrication. Then
a following expression is led easily from the expression (3) and the expression (4).
[0019] It is very common for the value of µ, Cox and Vth in the expressions (3) and (4)
to have dispersion among picture elements, among display apparatus or product lot,
but the expression (6) does not include these parameters, so that the value of the
Idrv/Iw does not depend on the dispersion of these parameters. If it is designed to
be W1 = W2 and L1 = L2, the value of the Idrv and the value of the Iw become the same
value, namely Idrv/Iw = 1. Namely the drive current Idrv flowing through the OLED
device is precisely accorded with the signal current Iw in spite of the dispersion
for the characteristics of these TFT devices, thereby a luminescence intensity of
the OLED device can be controlled precisely.
[0020] As above described, the Vth1 of the conversion transistor TFT 1 and the Vth2 of the
drive transistor TFT 2 are basically same, so that both the transistors TFT 1 and
TFT 2 are to be made OFF when a signal voltage for cutting off is supplied to respective
gate of both transistors TFT 1 and TFT 2. But practically due to the dispersion of
parameters in respective picture element, sometimes the Vth2 goes down below the Vth1.
In this case, a leakage current corresponding to the sub-threshold level flows through
the drive transistor TFT 2, so that the OLED device shows a minute luminescence. Because
of this minute luminescence, contrast of the displayed image is lowered and the display
characteristics are deteriorated. According to the present invention, it is particularly
set to be that the threshold voltage Vth2 of the drive transistor TFT 2 does not become
lower than the threshold voltage Vth1 of the corresponding conversion transistor TFT
1 within the picture element. For example the gate length L2 of the drive transistor
TFT 2 is set to be longer than the gate length L1 of the conversion transistor TFT
1 in order to attain that the threshold voltage Vth2 of the drive transistor TFT 2
does not become lower than the threshold voltage Vth1 of the corresponding conversion
transistor TFT 1. Thereby it is possible to suppress the above mentioned minute leakage
current and minute luminescence.
[0021] An embodiment of the present invention will now be described by way of non-limitative
example with reference to the accompanying drawings, in which:
Fig. 1 is a circuit diagram showing one embodiment of a picture element drive circuit
of the present invention;
Fig. 2 is a graph chart showing a relation between a threshold voltage and a gate
length of a thin film transistor;
Fig. 3 is a sectional view showing a construction of a display apparatus of this invention;
Fig. 4 is a waveform chart showing waveforms each signal in the picture element drive
circuit depicted in Fig. 1;
Fig. 5 is a block diagram showing a construction example ofthe display apparatus to
which the picture element drive circuit of Fig. 1 is applied;
Fig. 6 is a conventional picture element drive circuit; and
Fig. 7 is a block diagram showing a construction example of a conventional display
apparatus to which the picture element drive circuit of Fig. 6 is applied.
[0022] Fig. 1 is an example of a picture element drive circuit according to the present
invention. In this figure, the picture element drive circuit includes a conversion
thin film transistor TFT 1, where the signal current flows through the transistor
TFT 1, and a drive thin film transistor TFT 2 for controlling the drive current flowing
through a light emitting device consisting of an organic electro-luminescence device.
In addition, the picture element drive circuit further includes an input thin film
transistor TFT 3 for connecting and disconnecting the picture element drive circuit
to/from a data line DATA consisting of Mo-Ta in accordance with a control signal supplied
from a first scan line SCAN-A consisting of A1, a switch thin film transistor TFT
4 for connecting a gate electrode and a drain electrode of the conversion transistor
TFT 1 in accordance with a control signal supplied from a second scan line SCAN-B
consisting of Mo-Ta during writing period, a capacitor C having a structure the same
as a metal oxide semiconductor structure of the TFT 1 for maintaining a gate-source
voltage of the conversion transistor TFT 1 after completion of the writing period
and a light emitting device OLED (Organic Light Emitting Device). In the case of Fig.
1, the input transistor TFT 3 is an NMOS (N-channel Metal Oxide semiconductor) transistor
and the other transistors are PMOS (P-channel Metal Oxide Semiconductor) transistors,
but those are not limitation of a scope of the invention. As for the capacitor C,
one of terminals is connected to a gate electrode of the conversion transistor TFT
1 and the other terminal is connected to a potential Vdd (power potential), but it
is not limited to the power potential Vdd and any arbitrary fixed potential is available.
A cathode electrode of the light emitting device OLED is connected to ground potential.
[0023] The display apparatus of the present invention basically comprises a scanning line
drive circuit for sequentially selecting scanning lines SCAN-A and SCAN-B, a data
line drive circuit including a current source CS for generating a signal current Iw
having current level corresponding to intensity information and for supplying the
signal current Iw sequentially to the data line DATA and a plurality of picture elements
including current drive type light emitting device OLED provided at crossing portions
of each scanning lines SCAN-A, SCAN-B and each data line DATA for emitting light in
accordance with the received drive current. As a special feature matter, the picture
element as shown in Fig. 1 comprises an accept section for accepting the signal current
Iw from the corresponding data line DATA when the corresponding scanning line SCAN-A
is selected, a converting section for converting the accepted signal current Iw once
into corresponding voltage level and restoring the voltage level and a drive section
for supplying the drive current having current level corresponding to the restored
for supplying the drive current having current level corresponding to the restored
voltage level to the corresponding light emitting device OLED. The above mentioned
accept section consists of the input transistor TFT 3 to be concrete. Further the
converting section includes the conversion thin film transistor TFT 1 having, as above
mentioned, the gate electrode, the source electrode, the drain electrode and a channel
and the capacitor C connected to the gate electrode of the transistor TFT 1.
[0024] The conversion thin film transistor TFT 1 generates at the gate electrode the voltage
converted by flowing through the channel the signal current Iw taken and the capacitor
C restores the voltage thus generated at the gate electrode of the transistor TFT
1. Further the above mentioned accept section includes the switch thin film transistor
TFT 4 interposed between the drain electrode and the gate electrode of the conversion
thin film transistor TFT 1. This switch thin film transistor TFT 4 is made ON when
the current level of the signal current Iw is converted into the voltage level, and
generates at the gate electrode of the conversion thin film transistor TFT 1 the voltage
referenced with the source electrode by electrically connecting the gate electrode
and the drain electrode of the conversion thin film transistor TFT 1. In addition,
the switch thin film transistor TFT 4 is made OFF when restoring the voltage in the
capacitor C and the transistor TFT 4 disconnects the gate electrode of the conversion
thin film transistor TFT 1 and the capacitor C connected thereto from the drain electrode
of the conversion thin film transistor TFT 1.
[0025] Furthermore, the above mentioned drive section contains the drive thin film transistor
TFT 2 including the gate electrode, the drain electrode, the source electrode and
a channel. The drive thin film transistor TFT 2 supplies, through the channel, the
drive current to the light emitting device OLED, wherein the drive current has the
current level corresponding to the voltage level stored in the capacitor C and accepted
at the gate electrode of the transistor TFT 2. A current mirror circuit is constituted
by directly connecting the gate electrode of the drive thin film transistor TFT 2
to the gate electrode of the conversion thin film transistor TFT 1, so that the current
level of the signal current Iw and the current level of the drive current are made
to be a proportional relation. In this case, the drive thin film transistor TFT 2
works in the saturation range, and the transistor TFT 2 flows the drive current corresponding
to the difference between the voltage level given to the gate electrode and the threshold
voltage to the light emitting device OLED.
[0026] As another special feature matter of this invention, the threshold voltage of the
drive thin film transistor TFT 2 is set not to become lower than the threshold voltage
of the conversion thin film transistor TFT 1 within the picture element. To be more
concrete, a gate length of the transistor TFT 2 is set not to be shorter than a gate
length of the transistor TFT 1. Or thickness of a gate insulating film of the transistor
TFT 2 may be set not to be thinner than thickness of a gate insulating film of the
transistor TFT 1 corresponding to the picture element. Further the threshold voltage
of the transistor TFT 2 may be set not to be lower than the threshold voltage of the
transistor TFT 1 within the picture element by adjusting impurity density injected
in the channel of the transistor TFT 2 in the process of fabrication. If the threshold
voltage Vth1 of the conversion transistor TFT 1 and the threshold voltage Vth2 of
the drive transistor TFT 2 are set to be same, both the transistors TFT 1 and TFT
2 are to be made OFF when a signal voltage for cutting off is supplied to commonly
connected gate electrodes of both transistors TFT 1 and TFT 2. But practically due
to the dispersion of process parameters in respective picture element, there occurs
the case where the threshold voltage Vth2 of the transistor TFT 2 goes down below
the threshold voltage Vth1 of the transistor TFT 1. In this case, a leakage current
corresponding to a sub-threshold level flows through the drive transistor TFT 2 even
by the signal voltage of below the cut off level, so that the OLED device shows a
minute luminescence and contrast of the displayed image is lowered. Accordingly in
the present invention, the gate length L2 of the drive transistor TFT 2 is set to
be longer than the gate length L1 of the conversion transistor TFT 1. Thereby even
if the process parameters of the thin film transistor change within the picture element,
the threshold voltage Vth2 of the transistor TFT 2 does not become lower than the
threshold voltage Vthl of transistor TFT 1.
[0027] Fig. 2 is a graph chart showing a relation between a threshold voltage Vth and a
gate length L of a thin film transistor. In a short-channel effect area A where the
gate length L is relatively short, the threshold voltage Vth becomes high as the gate
length L increases. On the other hand in a suppression area B where the gate length
L is relatively long, the threshold voltage Vth is almost fixed in spite of the gate
length L. By utilizing this characteristic, the gate length L2 of the transistor TFT
2 is made longer than the gate length L1 of the transistor TFT 1 in this invention.
For example, the gate length L1 of the transistor TFT 1 is set to be 7 µm, then the
gate length L2 of the transistor TFT 2 is set to be about 10 µm. The gate length L1
of the transistor TFT 1 belongs to the short-channel effect area A, and the gate length
L2 of the transistor TFT 2 belongs to the suppression area B. Thereby, not only the
short channel effect in the transistor TFT 2 can be suppressed, but also it is possible
to suppress Accordingly, the minute luminescence of the OLED device is restrained
by suppressing the leakage current of the sub-threshold level flowing through the
transistor TFT 2, thereby this can contribute to the contrast improvement of the active
matrix type display apparatus. To be more concrete, when mask patterns are designed
for fabrication, this idea is taken in consideration, so that the gate length L2 of
the transistor TFT 2 is set to be longer than the gate length L1 of the transistor
TFT 1 without requiring any extra fabrication process.
[0028] Fig. 3 is a sectional view showing a construction of the display apparatus of this
invention. Only the OLED device and the transistor TFT 2 are depicted in Fig. 3 for
simplicity. The OLED device is formed by sequentially superimposing a reflection electrode
10 made of Mg-Ag, for example, an organic EL layer 11 and a transparent electrode
12 made of ITO (Indium Tin Oxide). The reflection electrode 10 is separated by one
picture element and functions to be the anode electrode of the OLED device. Each of
the transparent electrode 12 is commonly connected between the picture elements and
functions to be the cathode electrode of the OLED device. Namely each of the transparent
electrode 12 is commonly connected to the predetermined power potential Vdd. The organic
EL layer 11 is a complex film formed by superimposing a positive hole transport layer
and an electron transport layer. Diamyne is evaporated on the transparent electrode
10 functioning as the anode electrode (a positive hole injection electrode), Alq3
is evaporated thereon as the electron transport layer and finally the transparent
electrode 12 is formed on the Alq3 functioning as the cathode electrode (an electron
injection electrode). The above mentioned Alq3 represents an 8-hydroxy quinoline aluminum.
The OLED device having such laminated structure is only one example and this invention
is not limited by the depicted structure. When a forward direction voltage of around
10V is supplied between the anode electrode and the cathode electrode of the OLED
device having configuration as above described, injection of carriers such as the
electron or the positive hole occurs and the luminescence is observed. The luminescent
operation of the OLED device is thought to be based on an excitation formed by both
the positive hole injected from the positive hole transport layer and the electron
injected from the electron transport layer.
[0029] On the other hand the transistor TFT 2 comprises of a gate electrode 2 consisting
of Mo-Ta formed on a glass substrate 1, a gate insulating film 3 formed thereon and
consisting of SiO
2 and a semiconductor thin film 4 formed on the gate insulating film 3 and above the
gate electrode 2. This semiconductor thin film 4 consists of a polycrystalline silicon
thin film re-crystallized by a laser. The transistor TFT 2 equips with a source S,
a channel Ch and a drain D served as a transistor TFT 2 equips with a source S, a
channel Ch and a drain D served as a passage of the current to be supplied to the
OLED device. The channel Ch is positioned just above the gate electrode 2. The transistor
TFT 2 of this bottom gate structure is covered with an inter-layer insulating film
5 consisting of a PSG (Phosphosilicate Glass), for example, and a source electrode
6 and a drain electrode 7 respectively consisting of A1 are formed thereon. The OLED
device as above described is formed thereon by way of another inter-layer insulating
film 9 consisting of SiN. In the embodiment of Fig. 3, a P-channel thin film transistor
is formed as the transistor TFT 2, because the anode electrode of the OLED device
is to be connected to the drain electrode of the transistor TFT 2.
[0030] The gate length L2 of the transistor TFT 2 is set to become longer than the gate
length L1 of the transistor TFT 1. Or thickness d of the gate insulator 3 of the transistor
TFT 2 may set to become thicker than thickness of the gate insulator of the transistor
TFT 1. A threshold voltage of a thin film transistor becomes larger as thickness of
a gate insulator becomes thicker. To be more concrete, when the thickness of the gate
insulator of the transistor TFT 1 is set to be 200 nm, the threshold voltage can be
adjusted within several hundreds mv if the thickness d of the gate insulator 3 of
the transistor TFT 2 is set to be 220 nm. In this case, adjustment of the thickness
of the gate insulator may be done by etching process and photolithography. In some
cases, the threshold voltage may be adjusted by selectively injecting impurity in
the channel Ch of the transistor TFT 2. In case where the transistor TFT 2 is a P-channel
type, an impurity of P or As are selectively injected into the channel Ch in order
to shift the threshold voltage Vth2 toward the enhancement side. The constructions
of the transistors TFT 1, TFT 3 and TFT 4 are basically the same as the transistor
TFT 2 except that the OLED device, the organic EL layer and transparent electrode
are not provided.
[0031] Next, with reference to Fig. 4, a drive method of the picture element drive circuit
depicted in Fig. 1 is briefly explained. First of all, the first scanning line SCAN-A
and the second scanning line SCAN-B are set to be selected status when writing. In
the case of Fig. 4, the first scanning line SCAN-A is set to be low level and the
second scanning line SCAN-B is set to be high level. The signal current Iw corresponding
to the intensity information flows through the transistor TFT 1 by connecting the
current source CS to the data line DATA while both scanning lines SCAN-A and SCAN-B
are in the selected condition. The current source CS is a variable current source
controlled in accordance with the intensity information. In this time, the previously
mentioned expression (5) is established because the gate-drain of the transistor TFT
1 is short-circuited by the transistor TFT 4, so that the transistor TFT 1 works in
the saturation range. Accordingly the voltage Vgs given by the expression (3) occurs
between the gate-source of the transistor TFT 1. Next, the first scanning line SCAN-A
and the second scanning line SCAN-B are set to be non-selected status. Namely in more
detail, the transistor TFT 4 is set to be OFF condition by setting the second scanning
line SCAN-B to be low level. Thereby the voltage Vgs is restored in the capacitor
C. Then the picture element drive circuit is electrically disconnected from the data
line DATA by making the transistor TFT 3 to OFF condition by setting the first scanning
line SCAN-A to be high level, so that the writing to the other picture element drive
circuit can be possible after-words through the data line DATA. The data to be outputted
as current level of the signal current by the current source CS has to be effective
when the second scanning line SCAN-B is in non-selected condition, but afterwards
may be an arbitrary level (the writing data for the next picture element, for example).
The gate and source electrodes of the transistor TFT 2 are commonly connected to the
source electrodes of the transistor TFT 1, and those electrodes are formed closely
to each other within the small picture element circuit, so that the current flowing
through the transistor TFT 2 is determined by the expression (4) if the transistor
TFT 2 works in the saturation range. This current determined by the expression (4)
becomes the drive current Idrv flowing through the OLED device. In order to work transistor
TFT 2 in the saturation range, it is only to supply sufficient power potential as
the power voltage Vdd so as to establish the expression (5) even considering the voltage
drop at the OLED device.
[0032] Fig. 5 is a block diagram showing a construction example of the display apparatus
to which the picture element drive circuit of Fig. 1 is applied. The operation of
the display apparatus is explained as follows. First of all, a vertical start pulse
(VSP) is supplied to the scanning line drive circuit A21 constituting of thin film
transistors and including a shift register and to the scanning line drive circuit
B23 constituting of thin film transistors and including a shift register. These scanning
line drive circuits A21 and B23 select the first scanning line scanning line SCAN-A1∼SCAN-AN
and the second scanning line SCAN-B1∼ SCAN-BN sequentially in synchronism with vertical
clocks (VCKA, VCKB) after receiving the vertical start pulse (VSP). The current source
CS is provided in the data line drive circuit 22 constituting of thin film transistors,
and the current source CS drives the data line DATA with the current level corresponding
to the intensity information. The current source CS is constituting of a voltage-current
converting circuit as briefly depicted in a circle in Fig. 5 and outputs the signal
current in response to the voltage representing the intensity information. The signal
current flows to the picture element on the selected scanning line and is written
by the scanning line unit. Each of the picture elements starts luminescence by the
strength corresponding to the current level. In this case, the vertical clocks VCKA
are slightly delayed relative to the vertical clocks VCKB by a delay circuit 24. Thereby,
the second scanning line is set to be non-selected condition in advance of the first
scanning line scanning line.
1. An active matrix type display apparatus comprising:
a scanning line drive circuit for sequentially selecting scanning lines;
a data line drive circuit containing a current source for generating signal current
having current level corresponding to an intensity information and for sequentially
supplying thus generated signal current to data lines; and
a plurality of picture elements provided at each cross point of said data line and
said scanning line and each of the picture elements having a current drive type light
emitting device which emits light in response to drive current, wherein
each of said picture element comprises:
an accept section for accepting said signal current from the corresponding data line
when corresponding scanning line is selected;
a converting section for converting a current level of thus accepted signal current
once into corresponding voltage and restoring the converted voltage; and
a drive section for supplying the drive current having current level corresponding
to the restored voltage to the corresponding light emitting device,
said converting section includes: a conversion thin film transistor having a gate
electrode, a source electrode, a drain electrode and a channel; and
a capacitor connected to said gate electrode of the conversion thin film transistor,
wherein said conversion thin film transistor generates at the gate electrode the voltage
converted by flowing through said channel the signal current taken through said accept
section and said capacitor holds the voltage generated at the gate electrode, said
drive section contains:
a drive thin film insulated gate type field effect transistor including a gate electrode,
a drain electrode, a source electrode and a channel, wherein said drive thin film
insulated gate type field effect transistor supplies the drive current through the
channel to the light emitting device and the drive current has the current level corresponding
to the voltage restored in said capacitor and accepted at the gate electrode of the
drive thin film insulated gate type field effect transistor, and
a threshold voltage of said drive thin film insulated gate type field effect transistor
is set not to become lower than a threshold voltage of said conversion thin film insulated
gate type field effect transistor corresponding to the picture element.
2. A picture element drive circuit to be provided at each cross point of a data line
for supplying a signal current having current level corresponding to an intensity
information and a scanning line for supplying a selecting pulse and for driving a
current drive type light emitting device which emits light by a drive current, comprising:
an accept section for accepting said signal current from the corresponding data line
in response to said selecting pulse from said scanning line;
a converting section for converting thus accepted signal current once into corresponding
voltage and restoring thus converted voltage; and
a drive section for supplying the drive current having current level corresponding
to the restored voltage to the corresponding light emitting device, and said converting
section includes:
a conversion thin film transistor having a gate electrode, a source electrode, a drain
electrode and a channel; and
a capacitor connected to said gate electrode of the conversion thin film transistor,
wherein the conversion thin film transistor generates at the gate electrode the voltage
converted by flowing through said channel the signal current taken through said accept
section and said capacitor holds the voltage generated at the gate electrode of the
conversion thin film transistor,
said drive section contains:
a drive thin film insulated gate type field effect transistor including a gate electrode,
a drain electrode, a source electrode and a channel and the drive thin film insulated
gate type field effect transistor, wherein the drive thin film insulated gate type
field effect transistor supplies the drive current through the channel to the light
emitting device and the drive current has the current level corresponding to the voltage
restored in the capacitor and accepted at the gate electrode of the drive thin film
insulated gate type field effect transistor, and
a threshold voltage of said drive thin film insulated gate type field effect transistor
is set not to become lower than a threshold voltage of said conversion thin film insulated
gate type field effect transistor corresponding to the picture element.
3. An apparatus according to claim 1 or a circuit according to claim 2, wherein a gate
length of said drive thin film insulated gate type field effect transistor is set
not to be shorter than a gate length of said conversion thin film insulated gate type
field effect transistor within one picture element.
4. An apparatus or a circuit according to any one of the preceding claims, wherein a
thickness of a gate insulator of said drive thin film insulated gate type field effect
transistor is set not to be thinner than thickness of a gate insulator of said conversion
thin film insulated gate type field effect transistor within one picture element.
5. An apparatus or a circuit according to any one of the preceding claims, wherein
a threshold voltage of said drive thin film transistor is set not to be lower than
a threshold voltage of said conversion thin film insulated gate type field effect
transistor within one picture element by adjusting impurity density injected in said
channel of the drive thin film insulated gate type field effect transistor.
6. An apparatus or a circuit according to any one of the preceding claims, wherein said
drive thin film insulated gate type field effect transistor works in saturation range
and supplies the drive current corresponding to the difference between the threshold
voltage and the voltage given to the gate electrode into the light emitting device.
7. An apparatus or a circuit according to any one of the preceding claims, wherein a
current mirror circuit is constituted by directly connecting the gate electrode of
said drive thin film insulated gate type field effect transistor to the gate electrode
of the conversion thin film insulated gate type field effect transistor, so that the
current level of the signal current and the current level of the drive current are
made to be a proportional relation.
8. An apparatus or a circuit according to any one of the preceding claims, wherein said
accept section includes a switch thin film insulated gate type field effect transistor
interposed between the drain electrode and the gate electrode of the conversion thin
film insulated gate type field effect transistor,
said switch thin film insulated gate type field effect transistor is made ON when
the current level of the signal current is converted into the voltage and generates
at the gate electrode of the conversion thin film insulated gate type field effect
transistor said voltage referenced with the source electrode by electrically connecting
the gate electrode and the drain electrode of the conversion insulated gate type field
effect thin film transistor, and said switch thin film insulated gate type field effect
transistor is made OFF to disconnect the gate electrode of the conversion thin film
insulated gate type field effect transistor and the capacitor when restoring the voltage
to said capacitor.
9. An apparatus or a circuit according to any one of the preceding claims, wherein said
light emitting device is an organic electro-luminescence device.
10. An apparatus or a circuit according to any one of the preceding claims, wherein said
source, drain and channel of both said drive thin film insulated gate type field effect
transistor and said conversion thin film insulated gate type field effect transistor
are formed with poly-crystal semiconductor thin films.
11. A method for driving picture element to be provided at each cross point of a data
line for supplying a signal current having current level corresponding to an intensity
information and a scanning line for supplying a selecting pulse and for driving a
current drive type light emitting device which emits light by a drive current, comprising
the steps of:
step for accepting said signal current from the corresponding data line in response
to said selecting pulse from corresponding scanning line;
step for converting thus accepted signal current once into corresponding voltage and
restoring the voltage; and
step for driving by supplying the drive current having current level corresponding
to the restored voltage to the corresponding light emitting device, and
said converting step includes:
step for using a conversion thin film transistor having a gate electrode, a source
electrode, a drain electrode and a channel; and
a capacitor connected to said gate electrode of the conversion thin film transistor,
wherein the conversion thin film transistor generates at the gate electrode the voltage
converted by flowing through said channel the signal current taken through said accepting
step and said capacitor holds the voltage generated at the gate electrode,
said driving step includes:
step for using a drive thin film insulated gate type field effect transistor having
a gate electrode, a drain electrode, a source electrode and a channel, wherein said
the drive thin film insulated gate type field effect transistor supplies the drive
current through the channel to the light emitting device, wherein the drive current
has the current level corresponding to the voltage stored in the capacitor and accepted
at the gate electrode of the drive thin film insulated gate type field effect transistor,
and
a threshold voltage of said drive thin film insulated gate type field effect transistor
is set not to become lower than a threshold voltage of said conversion thin film insulated
gate type field effect transistor corresponding to the picture element.
12. A method according to claim 11, wherein
a gate length of said drive thin film insulated gate type field effect transistor
is set not to be shorter than a gate length of said conversion thin film insulated
gate type field effect transistor within one picture element.
13. A method according to claim 11 or 12, wherein
a thickness of a gate insulator of said drive thin film insulated gate type field
effect transistor is set not to be thinner than thickness of a gate insulator of said
conversion thin film insulated gate type field effect transistor within one picture
element.
14. A method according to any one of claims 11 to 13, wherein
a threshold voltage of said drive thin film transistor is set not to be lower than
a threshold voltage of said conversion thin film insulated gate type field effect
transistor within one picture element by adjusting impurity density injected in said
channel of the drive thin film insulated gate type field effect transistor.
15. A method according to any one of claims 11 to 14, wherein
said drive thin film insulated gate type field effect transistor works in saturation
range and supplies the drive current corresponding to the difference between the threshold
voltage and the voltage given to the gate electrode into the light emitting device.
16. A method according to any one of claims 11 to 15, wherein
a current mirror circuit is constituted by directly connecting the gate electrode
of said drive thin film insulated gate type field effect transistor to the gate electrode
of the conversion thin film insulated gate type field effect transistor, so that the
current level of the signal current and the current level of the drive current are
made to be a proportional relation.
17. A method according to any one of claims 11 to 16, wherein
said accepting step includes
a step for using a switch thin film insulated gate type field effect transistor interposed
between the drain electrode and the gate electrode of the conversion thin film insulated
gate type field effect transistor, wherein said switch thin film insulated gate type
field effect transistor is made ON when the current level of the signal current is
converted into the voltage and then generates at the gate electrode of the conversion
thin film insulated gate type field effect transistor said voltage referenced with
the source electrode by electrically connecting the gate electrode and the drain electrode
of the conversion insulated gate type field effect thin film transistor, and
said switch thin film insulated gate type field effect transistor is made OFF to disconnect
the gate electrode of the conversion thin film insulated gate type field effect transistor
and the capacitor when restoring the voltage to said capacitor.
18. A method according to any one of claims 11 to 17, wherein said light emitting device
is an organic electro-luminescence device.
19. A method according to any one of claims 11 to 18, wherein
said source, drain and channel of both said drive thin film insulated gate type field
effect transistor and said conversion thin film insulated gate type field effect transistor
are formed with poly-crystal semiconductor thin films.
20. An active matrix type display apparatus comprising:
a scanning line drive circuit for sequentially selecting scanning lines;
a data line drive circuit for sequentially supplying signal current corresponding
to an intensity information to data lines; and
a plurality of picture elements provided at each cross point of said data line and
said scanning line and each of the picture elements having a current drive type light
emitting device which emits light in response to drive current corresponding to said
signal current, wherein
each of said picture element comprises:
an input thin film transistor connected to said data line;
a conversion thin film transistor connected to said input thin film transistor for
converting said signal current on said data line to corresponding voltage;
a switch thin film transistor connected between a gate electrode and a source electrode
of said conversion thin film transistor,
a capacitor connected to said gate electrode of said conversion thin film transistor
for restoring said corresponding voltage; and
a drive thin film transistor connected to said light emitting device and to said capacitor,
wherein a threshold voltage of said drive thin film transistor is set not to become
lower than a threshold voltage of said conversion thin film transistor within one
the picture element.